Needless to say, the standard Scala collection library is very rich and provides collections for most of the common use cases. But of course, there are some other structures that might be useful in a number of specific situations. The Scala Collection Contrib module is Scala's way of having both a stable standard library and some extra features. In a sense, this module is an incubator for the new collection types; types that prove to be useful for a broad audience will presumably be incorporated into the standard library in further Scala versions.

Currently, there are four collection types available in the module, each both mutable and immutable:

• MultiDict
• SortedMultiDict
• MultiSet
• SortedMultiSet

Also this library provides a possibility to define additional operations on existing collections via an implicit enrichment. The following import is required to make it available: 

import scala.collection.decorators._

And it delivers these methods:

• On Seq: intersperse and replaced
• On Map: zipByKey, join, zipByKeyWith, mergeByKey, mergeByKeyWith, fullOuterJoin, leftOuterJoin, and rightOUterJoing

Please consult the module documentation at https://github.com/scala/scala-collection-contrib for further details.